The present invention relates to magnetic iron oxide particles which have a spindle shape and a substantially uniform particle size and shape, and are substantially free of dendrites and have a large axial ratio (major axial diameter/minor axial diameter) and an excellent print-through characteristic, and a method of producing the same.
The particles of the present invention are magnetic iron oxide particles having a spindle particle-shape, and a substantially uniform particle size and shape, and being substantially free of dendrites. These particles will be referred to simply as "spindle-shaped magnetic iron oxide particles" hereinunder.
With the development of a smaller-sized and lighter-weight magnetic recording apparatuses, the necessity for a recording medium having a higher performance such as a magnetic tape and a magnetic disk has been increasing more and more.
In other words, a magnetic recording medium is required to have a high recording density, a high sensitivity and a high output characteristics.
The properties in which magnetic iron oxide particles are required to have in order to satisfy the said demands on a magnetic recording medium are a small particle size, a high coercive force and an excellent dispersibility.
To state this more concretely, it is necessary that magnetic iron oxide particles have as high coercive force as possible in order to provide a magnetic recording medium with a high recording density, a high sensitibity and a high output. This fact is obvious from, for example, the descriptions: "Since the improvement of magnetic tapes has been directed toward a high sensitivity and a high output, emphasis has been laid on enhancing the coercive force of acicular .gamma.-Fe.sub.2 O.sub.3 particles, . . . " on page 310 of DEVELOPMENT OF MAGNETIC MATERIAL AND TECHNOLOGY OF IMPROVING DISPERSIBILITY OF MAGNETIC POWDER (1982), published by K.K. Sogo Gijutsu Center, and "It is known that there is a relationship between the particle size of acicular .gamma.-Fe.sub.2 O.sub.3 and the noise of a magnetic recording tape, and if the particle size become finer, the noise of magnetic recording tape lower" on page 312 of the same publication.
DEVELOPMENT OF MAGNETIC MATERIAL AND TECHNOLOGY OF IMPROVING DISPERSIBILITY OF MAGNETIC POWDER also discloses on page 312 "The condition for high-density recording in a coating type tape is that the tape is capable of maintaining high output characteristics with a low noise with respect to a short-wave signal. To satisfy this condition, it is necessary that the magnetic recording medium has both high coercive force (Hc) and large residual magnetization (Br), and a thin coated film. As seen from the above, it is really necessary that the magnetic recording medium has both high coercive force (Hc) and large residual magnetization (Br) in order to obtain a high recording density and to satisfy these conditions, and accordingly the magnetic iron oxide particles are required to have a high coercive force, an excellent dispersibility in the vehicle and a high orientation and a high packing density in the coated film.
The residual magnetization (Br) in a magnetic recording medium depends upon the dispersibility of the magnetic iron oxide particles in the vehicle and the orientation and packing density of the magnetic iron oxide particles in the coated film, and in order to improve these properties, the magnetic iron oxide particles dispersed in the vehicle are required to have as large an axial ratio (major axial diameter/minor axial diameter) as possible, a uniform particle size and no inclusion of dendrites.
As well known, the magnitude of the coercive force of magnetic iron oxide particles depend upon the configurational anisotropy, crystalline anisotropy, strain anisotropy, exchange anisotropy, or the interaction thereof.
Acicular magnetite particles and acicular maghemite particles which are used as magnetic iron oxide particles at present produce a relatively high coercive force by utilizing the anisotropy derived from their shapes, namely, by increasing the axial ratio (major axial diameter/minor axial diameter).
The known acicular magnetite particles are obtained by reducing the starting material goethite particles at 250.degree. to 400.degree. C. in a reducing gas such as hydrogen, and the known acicular maghemite particles are obtained by further oxidizing the thus-obtained magnetite particles at 200.degree. to 300.degree. C. in air.
As described above, magnetic iron oxide particles which have a substantially uniform particle size, are substantially free of dendrites, and have a large axial ratio (major axial diameter/minor axial diameter) are now in the strongest demand. In order to obtain magnetic iron oxide particles provided with these properties, it is necessary that the starting material goethite particles have a substantially uniform particle size, are substantially free of dendrites and have a large axial ratio (major axial diameter/minor axial diameter).
As a method of producing goethite particles, which are the starting material, a method of producing acicular goethite particles by oxidizing a solution containing ferrous hydroxide particles which is obtained by adding more than an equivalent of an alkaline solution to a ferrous salt solution, the oxidation being carried out by blowing an oxygen-containing gas into the solution containing ferrous hydroxide particles at not higher than 80.degree. C. at pH of not less than 11 [Japanese Patent Publication No. 39-5610 (1964)], a method of producing spindle-shaped goethite particles by oxidizing an aqueous solution containing FeCO.sub.3 which is obtained by reacting an aqueous ferrous salt solution with an alkali carbonate, the oxidation being carried out by blowing an oxygen-containing gas into the aqueous solution containing FeCO.sub.3 [Japanese Patent Application Laid-Open (KOKAI) No. 50-80999 (1975)], and the like are conventionally known.
As the magnetic iron oxide particles having a high coercive force, the so-called Co-doped magnetic iron oxide particles and the so-called Co-coated magnetic iron oxide particles are conventionally known. The coercive force of these magnetic iron oxide particles are apt to be increased with the increase in the Co content. The Co-doped magnetic iron oxide particles are obtained by producing Co-containing goethite particles by adding a Co salt when the starting material goethite particles are produced, and reducing the thus-obtained Co-containing goethite particles to produce Co-containing magnetite particles or further oxidizing the magnetite particles, if necessary, to produce Co-containing maghemite particles. The Co-coated magnetic iron oxide particles are obtained by using the magnetite particles or maghemite particles obtained by reducing or further oxidizing, if necessary, the starting material goethite particles as a precursor and coating the surfaces of the precursor particles with a Co compound.
The Co-doped magnetic iron oxide particles have a high coercive force, but they are disadvantageous in that since Co diffuses in the crystals, the distribution of the coercive force is enlarged, thereby making the magnetic iron oxide particles thermally and temporally unstable. In contrast, the Co-coated magnetic iron oxide particles are thermally and temporally stable.
There is no end to the recent demand for the improvement of the properties of magnetic iron oxide particles. In addition to uniform particle size, absence of dendrites, high coercive force, thermal and temporal stability and large axial ratio, (major axial diameter/minor axial diameter), the improvement of the capacity of transferring a recording signal to the opposing magnetic layer, the so-called a print-through characteristic is strongly demanded.
The print-through characteristic has a tendency to be deteriorated as the magnetic iron oxide particles become finer, especially, when the particle size is not more than 0.3 .mu.m. As to this tendency, ELECTRONIC TECHNIQUE (1968), vol. 10, published by Nikkan Kogyo Shimbun, discloses on page 51 ". . . it is known that the transferring effect has an unfavorable tendency to be deteriorated in proportion to the lowering of a noise level due to the reduction in the particle size . . .". This tendency is a serious problem in the present when the finer and finer magnetic iron oxide particles are apt to be used with the demand for a high recording density, a high sensitivity and a high output characteristic.
In addition, in order to improve the print-through characteristic of the Co-coated magnetic iron oxide particles, it is necessary that the particle size distribution of the precursor particles is as uniform as possible and the axial ratio (major axial diameter/minor axial diameter) is as large as possible. To satisfy these conditions, the starting material goethite particles are also required to have as uniform particle size distribution as possible and as large an axial ratio (major axial diameter/minor axial diameter) as possible.
Furthermore, demand for abbreviation in resources and energy used has become increasingly stronger, and it is also demanded that magnetic iron oxide particles is industrially and economically advantageously produced.
As described above, magnetic iron oxide particles which have fine and uniform particle size, no inclusion of dendrites, thermal stability independent to change with time, a large axial ratio (major axial diameter/minor axial diameter) and an excellent print-through characteristic are now in the strongest demand. If the method disclosed in Japanese Patent Publication No. 39-5610 (1964) is adopted for producing the starting material goethite particles, acicular goethite particles having a large axial ratio (major axial diameter/minor axial diameter), particularly, not less than 10 are produced, but dendrites are included therein. As to the particle size, they cannot be said to be the particles having a uniform particle size, and the print-through characteristic of the magnetic iron oxide particles obtained by using these goethite particles are unsatisfactory.
On the other hand, if the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 50-80999 (1975) is adopted for producing the starting material goethite particles, spindle-shaped particles having a uniform particle size and including no dendrites are produced. However, the axial ratio (major axial diameter/minor axial diameter) is at most about 7, in other words, the production of particles having a large axial ratio (major axial diameter/minor axial diameter) are difficult, and this phenomenon becomes prominent as the major axis diameter of the particles produced is reduced. The print-through characteristic of the magnetic iron oxide particles and Co-coated magnetic iron oxide particles obtained by using these goethite particles are unsatisfactory, either.
Various attempts have conventionally been carried out to increase the axial ratio (major axial diameter/minor axial diameter) of spindle-shaped goethite particles. For example, there is a method of reducing the gas flow velocity at which an oxygen-containing gas is blown into a suspension containing FeCO.sub.3 which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate to about 0.1 to 2.0 cm/sec, as disclosed in [Japanese Patent Application Laid-Open (KOKAI) No. 59-232922 (1984)]. According to this method, when the major axis is about 0.5 .mu.m, the axial ratio (major axial diameter/minor axial diameter) is about 10; when the major axis is about 0.3 .mu.m, the axial ratio (major axial diameter/minor axial diameter) is about 8; and when the major axis is as small as about 0.05 .mu.m, the axial ratio (major axial diameter/minor axial diameter) also becomes as small as about 5. The axial ratio (major axial diameter/minor axial diameter) cannot therefore be said to be sufficiently large.
In the example in the Japanese Patent Application Laid-Open (KOKAI) No. 62-158801 (1987), spindle-shaped goethite particles having an axial ratio (major axial diameter/minor axial diameter) of 10 are obtained. These particles, however, are obtained by making the concentration of iron as low as about 0.2 mol/l, and the axial ratio (major axial diameter/minor axial diameter) cannot be said to be sufficiently large, either.
Also, spindle-shaped magnetic iron oxide particles are disclosed in European Patent No. 160,496.
For these reasons, means for obtaining spindle-shaped magnetic iron oxide particles or Co-coated magnetic iron oxide particles which have a substantially uniform particle size, are substantially free of dendrites, and have a large axial ratio (major axial diameter/minor axial diameter) and an excellent print-through characteristic is now strongly demanded.
The present inventor made various researches so as to obtain spindle-shaped magnetic iron oxide particles or Co-coated magnetic iron oxide particles which have a substantially uniform particle size and shape, are substantially free of dendrites, and have a large axial ratio (major axial diameter/minor axial diameter) and an excellent print-through characteristic. As a result, it has been found that (1) spindle-shaped goethite are produced by aging at a temperature of 40.degree. to 60.degree. C. for 50 to 100 minutes an aqueous solution containing FeCO.sub.3 which is obtained by reacting an aqueous alkali carbonate with an aqueous ferrous salt solution in a non-oxidizing atmosphere, the amount of aqueous alkali carbonate being 1.5 to 3.5 times the equivalent based on Fe.sup.2+ in the aqueous ferrous salt solution, and blowing an oxygen-containing gas into the suspension containing FeCO.sub.3 for oxidation, and the thus-obtained spindle-shaped goethite particles or hematite particles obtained by baking these goethite particles are then reduced under heating in a reducing gas, or further oxidized, if necessary, thereby obtaining magnetic iron oxide particles of spindle-shaped magnetite (FeOx.multidot.Fe.sub.2 O.sub.3, 0&lt;x.ltoreq.1) particles having a print-through characteristic of not less than 45 dB or spindle-shaped maghemite particles having a print-through characteristic of not less than 53 dB, both of which have a major diameter of 0.1 to 0.3 .mu.m and an axial ratio (major axial diameter/minor axial diameter) of not less than 7; (2) spindle-shaped goethite particles containing zinc are produced by adding a zinc compound in advance to any of the aqueous alkali carbonate, the aqueous ferrous salt solution, the suspension containing FeCO.sub.3 and the suspension containing FeCO.sub.3 in the course of aging prior to the oxidation step of blowing the oxygen-containing gas thereinto, and the thus-obtained zinc-containing goethite particles or hematite particles obtained by baking these goethite particles are reduced under heating in a reducing gas, or further oxidized, if necessary, thereby obtaining spindle-shaped zinc-containing magnetite particles having a print-through characteristic of not less than 45 dB or spindle-shaped zinc-containing maghemite particles having a print-through characteristic of not less than 53 dB, both of which have a major diameter of 0.1 to 0.3 .mu.m and an axial ratio (major axial diameter/minor axial diameter) of not less than 8; (3) a mixed dispersion obtained by mixing an aqueous dispersion of the thus-obtained spindle-shaped maghemite containing or not containing zinc with at least an aqueous Co salt solution and an aqueous alkaline solution and having a pH of not less than 11 is heated at a temperature of 50.degree. to 100.degree. C., thereby obtaining spindle-shaped magnetic iron oxide particles with the surfaces thereof modified by 0.5 to 15.0 atomic % of Co based on Fe and Co; and (4) 1 to 50% of an aqueous alkali hydroxide based on the aqueous alkali carbonate is added to any of the aqueous alkali carbonate, the suspension containing FeCO.sub.3 and the suspension containing FeCO.sub.3 in the course of aging prior to the oxidation step of blowing the oxygen-containing gas thereinto so that the total amount of aqueous alkali carbonate and aqueous alkali hydroxide becomes 1.1 to 2.5 times the equivalent based on Fe.sup.2+ in the aqueous ferrous salt solution and the aging is carried out at a temperature of 30.degree. to 60.degree. C. for 10 to 100 minutes to produce spindle-shaped goethite particles, and the thus-obtained goethite particles or hematite particles obtained by baking these goethite particles are reduced under heating in a reducing gas, or further oxidizing the product, if necessary, thereby obtaining spindle-shaped magnetic iron oxide particles.
The present invention has been achieved on the basis of these findings.